1. Field of the Invention
The present invention relates to a printed circuit board (PCB). More particularly, the present invention relates to multi-layered PCBs having strip lines and micro strip lines.
2. Description of the Related Art
Electronic devices typically include printed circuit boards (PCB) on which components that control the devices are mounted. Printed circuit boards (PCBs) are becoming smaller as the electronic devices are scaled-down to meet the demand for compact electronic devices, e.g., for portable electronic devices. Also, today's electronic devices have many functions and transmit data at high speeds. Therefore, today's PCBs are correspondingly complex.
A PCB can be classified as a single-sided PCB in which wire (circuit) patterns are formed on only one side of an insulating substrate, a dual-sided PCB in which wire (circuit) patterns are formed on both sides of an insulating substrate, and a multi-layered PCB in which wire (circuit) patterns are formed on a plurality of layers on a substrate. Single-sided PCBs were predominately used in the past because the configurations of the components and the wire (circuit) patterns connected to the components were relatively simple. However, dual-sided and multi-layered PCBs are now being widely used today because the circuitry of today's electronic devices is more complex, more highly integrated and more compact than in the past.
A multi-layered PCB has an internal structure consisting of two or more layers, an external structure consisting of two or more layers, and a synthetic resin layer, e.g., a prepreg layer, interposed between the internal and external structures. A power circuit, a ground circuit, and a signal circuit are formed on the layers of the internal structure. The synthetic resin layer insulates the internal and external structures from each other and at the same time attaches the internal and external structures to one another. The multi-layered PCB also has vias that extend through the layers, respectively. A wire (circuit) pattern on one layer is electrically connected to the wire (circuit) pattern on another layer by the vias.
Furthermore, a central processing unit or a power integration chip is typically mounted on a multi-layered PCB. Either of these components generates heat during its operation. Therefore, a heat dissipating element, namely a heat sink, is disposed over the heat-generating component on the PCB to dissipate the heat from the component.
Also, thermal interface material (TIM) having excellent thermal conductivity is interposed between the multi-layered PCB and the heat sink to transfer the heat generated by the central processing unit or power integration chip to the heat sink. The thermal interface material plays a very important role in the multi-layered PCB because components, such as semiconductor chips and transistors, will fail if the heat is not sufficiently dissipated by the heat sink. A cooling fan may also be provided to rapidly cool the heat sink.
Still further, multi-layered PCBs used in the low frequency band are affected by the shape or length of the wires of their circuit patterns. In particular, errors that occur in the circuitry and interference between signals transmitted through the wires tend to become worse as the frequency becomes higher. And with this in mind, the components to be mounted to the PCB must be laid out such that the voltage or phase, which is dependent on the disposition of the wires, and the relatively small wavelength of the signals do not create problems during operation.
Also, the signal lines of a typical multi-layered PCB must share the same ground voltage. In this respect, the ground voltage is provided by a ground voltage plate of the PCB. Therefore, an electric field can be generated between the signal lines and the ground voltage plate due to the ground voltage. However, other signal lines and components must be provided between the signal lines and the ground voltage layer. Therefore, the signal lines and the ground voltage layer present difficulties in incorporating these additional signal lines and components into the PCB.
In order to resolve the above-described problem, the ground voltage plate is disposed on the back of a dielectric substrate as facing away from the signal lines. That is, complicated wire (circuit) patterns are provided on both sides of a substrate. However, in this case, components between the signal lines and the ground voltage plate are affected by an alternating electric field characterized according to the configuration of the signal lines (conductive wiring) and the dielectric material of the substrate. The affect becomes more complicated as the frequency (Mhz) at which the PCB operates becomes higher. With this in mind, the dielectric material of the substrate interposed between the signal lines and the ground voltage plate should be perfectly uniform, and so the thickness and dielectric constant of the substrate should be perfectly uniform.
To this end, a PCB having a micro strip line was developed. In this type of PCB, a ground voltage layer is formed on a lower surface of a dielectric substrate, and signal lines are formed on the upper surface of the dielectric substrate. Therefore, the thickness and dielectric constant of the substrate are uniform. However, a PCB having a micro strip line poses problems because the ground voltage layer is provided on the lower surface of the substrate. Specifically, a PCB having a micro strip line suffers from a fringing phenomenon in which signals leak into the surrounding air and can produce undesired cross talk (in which the energy of a signal transmitted through a signal line leaks into an adjacent signal line). That is, a PCB having a micro strip line experiences significant transmission loss.
A PCB having a strip line was developed to overcome the shortcomings of the PCB having the micro strip line. In a PCB having a strip line, ground voltage plates are disposed above and below the signal lines (the wire pattern made up of the signal lines). Therefore, the signal lines are isolated and the electric field is distributed evenly across the layer of signal lines in a vertical direction (orthogonal to the PCB).
FIG. 1 is a schematic diagram of a conventional multi-layered PCB having a micro strip line. The conventional multi-layered PCB comprises a circuit board body 10, a power voltage layer 20, a first insulating plate 30, a multi-layered substrate 40, and a protective layer 50. The multi-layered substrate 40 comprises a ground voltage plate 42, a second insulating plate 44, and a plurality of signal lines 46. Three signal lines 46 are shown in FIG. 1 for the sake of clarity but four or more signal lines can be formed over the circuit board body 10.
The power voltage layer 20, the first insulating plate 30, the ground voltage plate 42, and the second insulating plate 44 are disposed on the circuit board body 10 in the foregoing order. The signal lines 46 are formed on the second insulating plate 44, and the protective layer 50 is adhered to the second insulating plate 44 and signal lines 46.
The circuit board body 10 comprises insulating material and is a multi-layered substrate of the type used for electronic packages, multi-chip modules, and computer mother boards. The power voltage layer 20 is a conductive layer for supplying electrical power to the signal lines 46.
The first insulating plate 30 is a of a dielectric material which isolates the power voltage layer 20 and the ground voltage plate 42 of the multi-layered substrate 40 from one another, and by which the power voltage layer 20 and the ground voltage plate 42 of the multi-layered substrate 40 are adhered to one another.
The multi-layered substrate 40 supports a plurality of semiconductor devices electrically connected to the signal lines 46 such that the electrical power provided by the power voltage layer 20 is transferred by the signal lines 46 to the semiconductor devices, and various other signals are transferred between the signal lines 46 and the semiconductor devices. The ground voltage layer 42 of the multi-layered substrate 40 comprises a conductive layer for grounding signal lines 46. The second insulating plate 44 has a uniform thickness and composition and is a dielectric layer which isolates the ground voltage plate 42 from the signal lines 46 and preserves the signals transmitted through the signal lines 46. The second insulating plate 44 is made of a synthetic resin, e.g., a prepreg. The thickness and dielectric constant of the second insulating plate 44 play a very important role in the transmission of signals in the multi-layered substrate 40. The protective layer 50 is a layer of photo solder resist and covers most of the multi-layered substrate 40. The protective layer 50 prevents the signal lines 46 from being oxidized, contaminated or damaged.
FIG. 2 is a schematic diagram of a conventional multi-layered PCB having a strip line. The multi-layered PCB having a strip line comprises a circuit board body 10, a power voltage layer 20, a first insulating plate 30, a multi-layered substrate 40, a third insulating plate 60, and a second ground voltage plate 70. The multi-layered substrate 40 comprises a first ground voltage plate 42, a second insulating plate 44, and a plurality of signal lines 46.
The circuit board body 10, the power voltage layer 20, the first insulating plate 30, and the multi-layer substrate 40 (i.e., the first ground voltage plate 42, the second insulating plate 44 and the signal lines 46) are similar to those of FIG. 1 and thus, will not be described in further detail.
The second ground voltage plate 70 shields the signal lines 46 from the effects of electromagnetic waves in the environment around the PCB. In addition, the second ground voltage plate 70 along with the first ground voltage plate 42 ensures that the electric field lines across the signal lines 46 are all vertical (orthogonal to the plane of the signal lines). Therefore, the second ground voltage plate 70 minimizes the leakage of the signals transmitted through the signal lines 46 into the surrounding air, and thereby ensures a stable circuit operation.
The third insulating plate 60 has a dielectric constant and a thermal conductivity which are different from those of the second insulating plate 44. The third insulating plate 60 serves to insulate the signal lines 46, attach the second ground voltage plate 70 to the second insulating plate 44 and signal lines 46, and to minimize the impedance of the signal lines 46 along with the second ground voltage plate 70.
That is, as compared to the signal lines of the PCB having a micro strip line of FIG. 1, the signal lines of the PCB of FIG. 2 (i.e., a PCB having a strip line) have low impedance. That is, signals can be transmitted with a higher fidelity and with a higher power through a multi-layered PCB having a strip line than through a corresponding multi-layered PCB having a micro-strip line. However, designing a PCB so as to have a strip line is more difficult than designing a corresponding PCB having a micro strip line.
This is because only the signal line (circuit) pattern at the uppermost layer has to be considered when designing a PCB having a micro strip line. On the other hand, it is almost impossible to obtain the desired impedance of the signal lines in a PCB having a strip line because the signal line (circuit) pattern is embedded in the dielectric of an insulating plate (the third insulating plate 60 in FIG. 2). In addition, the fabricating of a PCB having a strip line is more complicated because the signal line pattern is interposed between a pair of ground voltage plates (ground voltage plates 44 and 70 in FIG. 2). Therefore, a separate structure having the signal line (circuit) pattern should be fabricated and assembled to ground voltage plates.
The limitations of the multi-layered PCB having a strip line could be overcome by adding layers. However, the thickness of the PCB would be increased by the additional layers, the fabricating cost would be correspondingly higher, and the vias required for connecting the circuit patterns of the additional layers would compromise the signal transmission characteristics of the resulting PCB.
Of course, the width of the signal lines could be redesigned to overcome the limitations of the multi-layered PCB having a strip line. However, changes to the shape or length of the signal lines will change the impedance of the signal lines. That is, the impedance of a signal lines is dependent on the width of the signal line. In particular, the larger the width of a signal line is, the smaller the impedance. Thus, any changes to the design width of the signal lines may result in a circuit that operates abnormally, and may facilitate electro magnetic interference (EMI) in a high frequency band (resulting in poor reception), and cross talk.